Gray scale display device

ABSTRACT

The gradation display device contains gradient detecting circuit ( 3 ) for detecting a gradient of gradation values of pixels in an incoming image; time-varying gradation-value detecting circuit ( 4 ) for detecting changes in gradation values of the pixels with a passage of time; an image detector for detecting a magnitude and a direction of movement of the incoming image according to outputs from gradient detecting circuit ( 3 ) and time-varying gradation-value detecting circuit ( 4 ); and gradation correcting circuit ( 12 ) for correcting signals of the incoming image according to the detected magnitude and direction of the image and the weight of luminance assigned to each of the sub-fields so as to display proper image.

TECHNICAL FIELD

The present invention relates to a gradation display device usingsub-fields. More particularly, it relates to a gradation display devicecapable of decreasing gradation disturbances—known as dynamic falsecontours—when moving image is shown on the screen.

BACKGROUND ART

In a image display device employing sub-fields to display gradationlevels, such as a plasma display panel (PDP), image quality has oftendegraded by a noise generated in displaying moving image, known asdynamic false contours.

It is well known in those skilled in the art that the dynamic falsecontours can be suppressed by increasing the number of the sub-fields.In some kinds of the devices, such as PDPs, however, increase in thenumber of the sub-fields makes difficult to hold sufficient time foremission, resulting in lack of luminance. To address the problem above,some attempts have been made. For example, Japanese Patent UnexaminedPublication No. 2000-276100 suggests that the number of the sub-fieldsshould be kept relatively small and combinations of the sub-fieldscorresponding to the gradation level of an image to be shown should becontrolled in the area susceptible to the dynamic false contours toenhance both of the moving image quality and luminance.

Employing the method, the conventional device limits the number of thegradation levels fro image display in the area showing moving image, andshows image by using a combination of gradation values relativelyunsusceptible to the dynamic false contours; on the other hand, tomaintain consistent gradation levels, a dithering process producessubstantial gradation levels.

However, the conventional display device, detection of moving pictureswas not designed to precisely correspond to the gradation display methodemploying the sub-fields; it has been waited for improvement in accuratedetection in areas in which the dynamic false contours are prominentlyobserved, or likely to occur.

To address the problem above, the present invention provides a gradationdisplay device with a simple circuit structure, which can accuratelydetect the areas in which the dynamic false contours likely to occur.

DISCLOSURE OF THE INVENTION

To address the problem above, according to the gradation display deviceof the present invention, a TV field is divided into multiple sub-fieldseach of which has a predetermined weight of luminance. With the multiplesub-fields, the device provides gradation display. The device contains agradient detector for detecting a gradient of gray-scale values ofpixels of an image fed into the device; a time-varying gradation-valuedetector for detecting changes in the gradation values of pixels withthe passage of time; an image detector for detecting the magnitude anddirection of movement of the incoming image according to the outputsfrom the gradient detector and the time-varying gradation-valuedetector; and a signal corrector for correcting signals of the incomingimage according to the detected magnitude and direction of the image anda weight of luminance assigned to each sub-field so as to display properimage on the screen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the structure of the gradationdisplay device of an embodiment of the present invention.

FIG. 2 shows correction levels corresponding to characteristics ofimages and ranges.

FIG. 3 is a block diagram illustrating an example of a smoothnessdetecting circuit of the device.

FIG. 4 is a block diagram illustrating an example of a gradientdetecting circuit of the device.

FIG. 5 shows a pattern of coefficients used for a filter of the gradientdetecting circuit.

FIG. 6 is a block diagram illustrating a detection of time-varyinggradation values of the device.

FIG. 7 shows characteristics of an evaluation circuit of the device.

FIG. 8 shows how the final judge is obtained.

FIG. 9 illustrates how to calculate the amount of movement of an imagefrom the gradient and the time-varying gradation values.

FIG. 10 shows the characteristics of a gradation disturbance evaluatingcircuit of the device.

FIG. 11 illustrates the characteristics of a gradation correctingcircuit of the device.

FIG. 12 shows a combination of the weights of luminance and emissionassigned to each sub-field of the device.

FIG. 13 shows how to encode in an encoding circuit of the device.

FIG. 14 shows the relation between a direction of gradient in an imageappearing area and a moving direction of an image in a gradation displaydevice of another embodiment of the invention.

FIG. 15 shows evaluation of gradation disturbance of the device.

FIG. 16 is a block diagram illustrating the structure of a gradationdisplay device of still another embodiment of the invention.

FIG. 17 shows component VG in the direction of a gradient of movementvector V of the device.

FIG. 18 illustrates the structure of a gradation disturbance predictioncircuit of the device.

FIG. 19 is a block diagram illustrating the structure of a gradationdisplay device of yet another embodiment of the invention.

FIG. 20 is a block diagram illustrating the structure of the gradationcorrecting circuit of the device.

FIG. 21 illustrates a general error-variance coefficient.

FIG. 22 illustrates the control method of error-variance of the deviceof the invention.

FIG. 23 shows transition of error-variance coefficient EA of the device.

FIG. 24 shows how to calculate error-variance coefficient EA of thedevice.

FIG. 25 illustrates the interpolation image of error-variancecoefficient EA of the device.

FIG. 26 shows transition of error-variance coefficient EB of the deviceFIG. 27 illustrates the interpolation image of error-variancecoefficient EB of the device.

FIG. 28 illustrates the interpolation image of error-istributioncoefficient EC of the device.

FIG. 29 illustrates the interpolation image of error-variancecoefficient ED of the device.

DETAILED DESCRIPTION OF CARRYING OUT OF THE INVENTION

The gradation display device of an embodiment of the present inventionwill be described hereinafter with reference to the accompanyingdrawings.

First Exemplary Embodiment

FIG. 1 is a block diagram illustrating the structure of the gradationdisplay device of an embodiment of the present invention. In FIG. 1,image signals entered through input terminal 1 are fed into smoothnessdetecting circuit 2 as a smoothness detector, gradient detecting circuit3 as a gradient detector, and time-varying gradation-value detectingcircuit 4 as a time-varying gradation-value detector for detectingchanges in the gradation values of pixels with the passage of time.Smoothness detecting circuit 2 detects smoothness in the gradationvalues of pixels of an incoming image. Gradient detecting circuit 3detects a gradient in the gradation values of pixels in a display area.

The outputs from smoothness detecting circuit 2, gradient detectingcircuit 3, and time-varying gradation-value detecting circuit 4 arecompared with each predetermined threshold in evaluation circuits 5, 6,and 7, respectively. Receiving the outputs from evaluation circuits 5,6, and 7, final judge circuit 8 outputs final judge result k.

Evaluation circuit 5 has definable threshold TH1. Receiving output Sfrom smoothness detecting circuit 2, evaluation circuit 5 comparesoutput S with threshold TH1, and outputs judge result k1. Evaluationcircuit 6 has two definable thresholds TH2 and TH3. Receiving output Gfrom gradient detecting circuit 3, evaluation circuit 6 compares outputG with thresholds TH2 and TH3, and outputs judge result k2. Similarly,evaluation circuit 7 has two definable thresholds TH4 and TH5. Receivingoutput B from time-varying gradation-value detecting circuit 4,evaluation circuit 7 compares output B with thresholds TH4 and TH5, andoutputs judge result k3. Judge results k1, k2, and k3 are fed into finaljudge circuit 8.

Movement amount detecting circuit 9 receives output G from gradientdetecting circuit 3 and output B from time-varying gradation-valuedetecting circuit 4. According to the outputs, movement amount detectingcircuit 9 detects magnitude and direction of movement of an image to beentered. Gradation disturbance evaluating circuit 10 receives output Gfrom gradient detecting circuit 3 and output m1 from movement amountdetecting circuit 9. Receiving output m2 from gradation disturbanceevaluating circuit 10 and final judge result k from final judge circuit8, correction amount control circuit 11 outputs output m3, whichcontrols gradation correcting circuit 12 as a signal corrector.

Receiving image signals from input terminal 1 and output m3 fromcorrection amount control circuit 11, gradation correcting circuit 12outputs data to sub-field gradation display device 13. That is,according to the magnitude and direction of movement of an image(detected at movement amount detecting circuit 9) and a weight ofluminance assigned to the sub-field of an incoming image signal,gradation correcting circuit 12 corrects the image signal for displayingimage properly.

Hereinafter will be described in detail the workings of each section ofthe gradation display device.

In FIG. 1, according to each output from smoothness detecting circuit 2,gradient detecting circuit 3 and time-varying gradation-value detectingcircuit 4, evaluation circuits 5, 6, and 7 detect characteristics of atarget pixel or an image of a target area. FIG. 2 shows combinationpatterns of characteristics and corresponding correction control.

In FIG. 2, evaluation circuits 5, 6, and 7 receive the outputs fromdetecting circuits 2, 3 and 4, respectively, and compare the outputswith each threshold to determine the characteristics of the incomingimage. The results are further fed into final judge circuit 8, where thetarget area is put into one of the six groups: “no change with time”,“drastic change with time”, “smooth area”, “edge area”, “constantlyinclined area”, and “complicate pattern”. Final judge circuit 8determines final judge result k according to the group to which thetarget area is classified. An inequality sign in FIG. 2 represents therelation in magnitude between the characteristics of an image and athreshold. In each combination of outputs S, G, and B, “x” is given toan output that does not work as a key factor in the correction control.

Evaluation circuit 5 determines, as shown in FIG. 2, the range thatsatisfies S≧TH1 (where, S represents smoothness of the target area, TH1represents the threshold given to evaluation circuit 5). Evaluationcircuit 6 determines the range that satisfies TH2≦G≦TH3 (where, Grepresents gradient of the gradation value of the target area, TH2 andTH3 represent the thresholds given to evaluation circuit 6). Similarly,evaluation circuit 7 determines the range that satisfies TH4≦B≦TH5(where, B represents the changes with time in gradation values in thetarget area, TH4 and TH5 represent the thresholds of evaluation circuit7). Receiving the results above, final judge circuit 8 determines thepixels included in the range as the area in which the dynamic falsecontour is likely expected, or easily detected, and provides the areawith gradational correction for proper display.

The dynamic false contour is conspicuously observed in the area havingfollowing conditions: each of the gradient of gradation values of pixelsforming image and changes with time in gradation values of the pixelsstays in a range having a moderate upper limit and lower limit; and theimage pattern is relatively smooth. The device of the present inventionselectively detects such areas.

Now will be described each example of smoothness detecting circuit 2,gradient detecting circuit 3, and time-varying gradation-value detectingcircuit 4. First, smoothness detecting circuit 4 contains, as shown inFIG. 3, delay circuits 20, pixel averaging circuit 21, differentialcircuits 22, absolute value calculating circuits 23, and adder circuit24. Delay circuits 20 provide each pixel signal with a delay accordingto an image signal from input terminal 1; pixel averaging circuit 21receives the pixel signals from delay circuits 20 and averages thegradation values of the pixel signals; differential circuits 22 obtainthe difference between the gradation value of each pixel signal and theaverage value by calculating the difference between the output frompixel averaging circuit 21 and the outputs from delay circuits 20;absolute value calculating circuits 23 calculate the absolute values ofthe differential values obtained at differential circuits 22; and addercircuit 24 outputs smoothness of the gradation value of each pixel ofincoming image signals by adding the absolute values received fromabsolute value calculating circuits 23.

Gradient detecting circuit 3 contains, as shown in FIG. 4, horizontalfilter 30 for detecting horizontal changes in gradation values ofpixels, vertical filter 31 for detecting vertical changes in gradationvalues of pixels; absolute value calculating circuit 32 for calculatingeach absolute value of the outputs fed from filters 30 and 31; and addercircuit 33 for adding the two outputs from absolute value calculatingcircuit 32. Each of filters 30 and 31 multiplies the pixels adjacent tothe target pixel by a predetermined coefficient and then add the resultseach other. FIGS. 5A and 5B show examples of the coefficients used forthe filters. Receiving image signals from input terminal 1, horizontalfilter 30 and vertical filter 31 detect horizontal and vertical changesin the gradation value of pixels. Adding the absolute values of eachoutput from the filters can detect a gradient of the gradation value ofpixels of incoming image signals.

Time-varying gradation-value detecting circuit 4 contains, as shown inFIG. 6, field delay circuit 40, differential circuit 41, and absolutevalue calculating circuit 42. Field delay circuit 40 delays signalscorresponding to one field of incoming image signal. Differentialcircuit 41 calculates the difference between the gradation value ofpixels of current image signal and the gradation value of pixels ofone-field-before image signal fed from delay circuit 40. Absolute valuecalculating circuit 42 calculates the absolute value of the output fromdifferential circuit 41. With the structure above, time-varyinggradation-value detecting circuit 4 detects changes in the gradationvalue of target pixels with the passage of time by calculating thedifference between the gradation value of pixels of current image signaland the gradation value of pixels of one-field-before image signal.

Although FIG. 2 shows two levels-“correction: small”, “correction:large” for the sake of simplicity, the gradational correction of thedevice has multi-levels at least three. The device can continuouslyswitch the correction levels to provide smooth correction. FIGS. 7A, 7B,and 7C show the characteristics of evaluation circuits 5, 6, and 7,respectively. The characteristics of the circuits shown in FIGS. 7Athrough 7C can realize the smooth correction of the gradational display.

The working of evaluating circuits 5, 6, and 7 will be described withreference to the characteristics shown in FIG. 7A through FIG. 7C.

Receiving smoothness S fed from detecting circuit 2, evaluating circuit5 compares S with TH1 that is the threshold given to circuit 5. As shownin FIG. 7A, when S has a value close to TH1, the output of evaluationcircuit 5 takes a value between 0 and 1. When S is smaller than TH1, theoutput takes a value closer to 0, on the other hand, when S is greaterthan TH1, the output takes a value closer to 1.

Receiving gradient G fed from detecting circuit 3, evaluating circuit 6compares G with TH2 and TH3 that are the thresholds given to circuit 6.When G takes a value between TH2 and TH3, as shown in FIG. 7B, theoutput of evaluating circuit 6 takes a value closer to 1; otherwise, theoutput takes a value closer to 0.

Receiving output B (where, B represents the change with time of thegradation value) fed from detecting circuit 3, evaluating circuit 7compares B with TH4 and TH5 that are the thresholds given to circuit 7.When B takes a value between TH4 and TH5, as shown in FIG. 7C, theoutput of evaluating circuit 7 takes a value closer to 1; otherwise, theoutput takes a value closer to 0. It will be understood that each outputof evaluating circuits 5, 6, and 7 may take a shape with a steppedchange.

Final judge circuit 8 outputs final judge result k. Having multipliers81 and 82 in the structure, as shown in FIG. 8, final judge circuit 8calculates the product of k1, k2, and k3 fed from evaluating circuits 5,6, and 7, respectively. According to the characteristics of image fromcircuits 5 through 7, final judge circuit 8 properly outputs final judgeresult k.

On the other hand, magnitude of the movement of an image, i.e., theamount of the movement and the direction of the movement of the imageare detected in movement amount detecting circuit 9 according togradient G fed from gradient detecting circuit 3 and time-varyinggradation value B fed from time-varying gradation-value detectingcircuit 4. In theory, the calculation can be carried out by thefollowing method on the assumption that the gradation value of an imagechanges with the shape of the showing object maintained.

Based on the premise that the amount of movement of an image is, asshown in FIG. 9, in proportion to B (which represents the change withtime of the gradation value of the target area), and in inverseproportion to changes in gradation values in the screen, i.e., gradientB, the amount of movement of an image represented by m1 is obtained bythe expression: m1=B/G. However, the aforementioned assumption does nothold for an area in which gradient G has a great change, so that theamount of movement cannot be accurately detected. Similarly, in the areawhere gradient G is extremely small, the denominator of the expressiontakes a small value. In this case, too, an accurate detection cannot beprovided. Furthermore, when the change with time in gradation values isvery small, the dynamic false contour hardly occurs. In contrast, whenthe change with time in gradation values is considerably large, even ifthe dynamic false contour appears on the screen, it would hardly beperceptible as a dynamic false contour. Considering to the facts above,the limited combinations of characteristics of images (FIG. 2) enable toprovide an accurate detection of the movement of images in the areawhere the dynamic false contour is likely to occur. That is, correctionof the dynamic false contour according to output k fed from final judgecircuit 8 can accurately detect the movement of images and properlycorrect image signals.

The amount of movement through the calculation in movement amountdetecting circuit 9 can be accurately obtained as long as thecharacteristics of images satisfy the aforementioned conditions.However, the amount of movement detected here represents the number ofpixels per unit of time, which is a physical quantity essentially differfrom the dynamic false contour as a disturbance in gradational display.Besides, the detected amount may not completely agree with a visualevaluation of actually recognized dynamic false contour.

To provide more accurate detection, the device of the present inventioncontains gradation disturbance evaluating circuit 10 having dimensionalinput/output characteristics shown in FIG. 10. Gradation disturbanceevaluating circuit 10 determines disturbance in gradation valuesrepresented by m2. Receiving m1 (which represents the amount of movementof image, or the number of pixels per unit of time) fed from movementamount detecting circuit 9, evaluation circuit 10 converts m1 into m2and sends it to correction amount control circuit 11.

FIG. 10 shows that gradation disturbance evaluating circuit 10 hascharacteristics in which the dynamic false contour has a maximum valueat a mean value of the amount of movement when the amount of movement ischanged with respect to a constant gradient. That is, thecharacteristics of evaluating circuit 10 shows that the dynamic falsecontour intensely occurs in the area having a large amount of movementwith the gradient kept relatively small (such as at A of FIG. 10), andin the area having a large gradient with the amount of movement keptrelatively small (such as at B of FIG. 10).

Correction amount control circuit 11 is formed of, for example, amultiplier (not shown). Receiving m2 that represents calculateddisturbance in gradation values from circuit 10, correction amountcontrol circuit 11 outputs gradation correcting signal m3 as the productof m2 and final judge coefficient k.

Receiving gradation correcting signal m3, gradation correcting circuit12 performs gradational correction according to the structure ofsub-fields, movement of images, and gradation values, thereby minimizingdynamic false contours inevitably generated in image display employingsub-fields. Gradation correcting circuit 12 is formed of, as shown inFIG. 11, a combination of an encoding circuit and a feedback circuit.

In FIG. 11, an image signal from input terminal 1 is fed to encodingcircuit 122 via adder 121. In encoding circuit 122, the image signalundergoes a predetermined encoding process and goes out from outputterminal 125. In the process, subtracter 123 calculates the differencein the signal between pre-encoding and post-encoding. The difference isfed to feedback circuit 124 and then added to an input signal in adder121. In general, feedback circuit 124 contains a plurality of delayelements and coefficient circuits, and gradational control is carriedout in encoding circuit 122. That is, gradation correcting circuit 12performs an error-variance process.

FIG. 12 shows an encoding method used for gradation display device 13,which is formed of a combination of the weights of luminance andemission assigned to each sub-field. FIG. 12 introduces the combinationusing ten sub-fields of SF1 through SF10. The weighting ratio ofluminance assigned to the ten sub-fields are 1, 2, 4, 8, 16, 24, 32, 40,56, and 72. FIG. 12 shows an encoding method of sub-field assignmentcorresponding to the gradation value of an incoming image. In the table,numeral ‘1’ is given to a sub-field having emission.

FIG. 13 introduces an encoding method used in encoding circuit 122 ofFIG. 11, showing the weight of luminance assigned to the sub-fields andthe encoding method of the weighting. For a small amount of correction,the device performs gradation control using many gradation levels. Incontrast, for a large amount of correction, the device performsgradation control using fewer gradation levels, and at the same time,shows image using substantial gradation levels obtained byerror-variance. FIG. 13 shows eight levels of gradational correction of0-7. A dot is given to a gradation value to be used. The gradationcontrol is performed so that all the gradation levels can be used whenthe amount of gradational correction takes 0, and the number of thegradation levels is kept at a minimum when the amount of gradationalcorrection takes 7. With the gradation control above, the deviceprovides a larger amount of correction in an area where an intensedynamic false contour is expected, thereby maintaining the correlationbetween the gradational levels and luminance distribution of thesub-fields, which prevents the dynamic false contours. The deviceprovides a smaller amount of correction as the occurrence of the dynamicfalse contour decreases, allowing the image on the screen to have acontinuous gradational correction. As a result, the device can realize asmooth control for suppressing the dynamic false contour and propergradational correction also in an area having no dynamic false contours.

According to the embodiment of the present invention, as describedabove, the device contains a detecting unit for detecting magnitude anddirection of movement of incoming image according to a gradient of theimage in the screen and changes with time in gradation values; and asignal correcting unit for correcting an incoming image signal accordingto the magnitude and direction of the movement of the image and a weightof luminance assigned to the sub-fields. With the simple structure, thedevice can provide proper gradational display.

A conventionally well known method of calculating the movement itself ofimages from a gradient of the images and changes in gradation valueswith the passage of time is introduced, for example, in MultidimensionalSignal Processing for TV image, pp. 202-207, Takahiko Fukinuki, Nov. 15,1988. The gradient method described in the book above is effective inthe case where the movement of images is relatively small; it has not bewidely used in practice.

To address the pending problems above, the inventors examined thebehavior of the dynamic false contour generated in a display deviceemploying the sub-fields, and found how the structure of the sub-fields,characteristics of image, the movement amount of image affect on theoccurrence of the dynamic false contour. The analysis tells that thelocation and intensity of the dynamic false contour can be easilydetected as long as both of the gradient of gradation values and thechanges in gradation values with the passage of time stay in each rangehaving a predetermined upper limit and a lower limit. Besides, thegradient and the changes in gradation values allow the movement ofimages to be almost perfectly detected. Employing the detection above,the simply structured device of the present invention can offerexcellent visibility in both of moving image and still image.

Although the inventive concepts-weighting of luminance to thesub-fields, encoding the sub-fields, evaluating the amount of gradationdisturbance from the movement amount of image, correcting gradation, andthe like—have been shown and described above, it will be understood thatmany changes and modifications may be made.

Second Exemplary Embodiment

Here will be described another embodiment of the present invention. Forthe gradational control, the device of the present invention employs anamount of correction, which is acquired by totally evaluating thesmoothness of gradation values of an incoming image signal and thegradient of the gradation values, and changes in gradation values withthe passage of time. The structure of the embodiment focuses on therelation between the direction of the gradient of the gradation valuesand the direction of changes in gradation values with the passage oftime. With the structure, intensity of the dynamic false contour isfurther accurately detected for the proper image correction. Compared tothe structure shown in FIG. 1, the device of the embodiment has the samestructure, except for the internal structure and the working ofgradation disturbance evaluating circuit 10. The description will befocused on the difference.

FIG. 14 shows the relation between a direction of gradient in an imageappearing area and a moving direction of the image in the gradationaldisplay device of the embodiment. The table of FIG. 14 is the same asthat shown in FIG. 12 but for solid-lined arrow a and dot-lined arrow b.The two arrows illustrate the difference in amount of the dynamic falsecontour generated when a viewer watches an image that is moving oppositeto an image area where the gradient of gradation is uniform.

For example, suppose that a lamp waveform having a gradation value of200 as a mean value is moving in the screen. When an image moves in adirection opposite to the increasing direction of the gradation values(indicated by arrow a), the amount of emission of the sub-fields areobserved smaller than the amount should be actually measured. This leadsto a relatively intense dynamic false contour. In contrast, when animage moves along in the increasing direction of the gradation values(indicated by arrow b), the amount of emission of the sub-fields areobserved slightly larger than that should be actually measured; comparedto the movement in the opposite direction, however, the amount isrelatively small. As a result, the intensity of the dynamic falsecontour becomes relatively low.

In evaluating the intensity of the dynamic false contour from themovement of an image, as described above, further accurate imagecorrection can be obtained by changing the amount of image correctionaccording to the correlation between the moving direction of an imageand the direction of the gradient of gradation values in the screen.

FIG. 15 illustrates the control of the image correction above, showingthe magnitude and direction of movement of an image, and evaluation ofgradational disturbance with respect to the gradient of gradationvalues. The graph shows the function having two dependent variables,i.e., movement of image represented by the horizontal axis, and gradientof image represented by the vertical axis. A value defined by thefunction in a vertical upward direction from the surface of the paperrepresents an amount of gradation disturbance, that is, the evaluationvalue of the dynamic false contour.

The device of the embodiment, as is apparent from FIG. 15, changes theamount of image correction according to the combination of the movingdirection of an image and the direction of the gradient of the gradationvalues even when the gradient of an image and the movement of the imagehave an identical absolute value. The image correction shown in FIG. 15is so determined that the amount of image correction increases as theabsolute value of the moving amount of an image increases, and when theabsolute value takes a predetermined value, the amount of imagecorrection reaches the maximum. The maximum value depends on thecombination of the directions of image movement and the gradient ofgradation values. For example, the amount of image correction takes amaximum value when the combination of a positive (+) direction of imagemovement and a positive (+) direction of the gradient of gradationvalues; and when the combination of a negative (−) direction of imagemovement and a negative (−) direction of the gradient of gradationvalues.

According to the embodiment, to suppress the dynamic false contour, thedevice changes the amount of image correction according to thecombination of the moving direction of an image and the direction of thegradient of the gradation values, which enables an excellent gradationaldisplay with a simple structure.

Third Exemplary Embodiment

Here will be described still another embodiment of the present inventionwith reference to FIGS. 16 through 18.

The gradational display device of the embodiment separately detects thehorizontal component and the vertical component of a direction ofmovement of an image, and converts the gradient and movement of an imageinto a component in a direction of the gradient, thereby providingsignal correction. In FIG. 16, like parts are identified by the samereference marks as in FIG. 1, and the description thereof will beomitted.

In FIG. 16, gradient detecting circuit 31 outputs the absolute value ofgradient of gradation values represented by |G|, gradient horizontalcomponent Gx and gradient vertical component Gy. Receiving output B(representing the change in gradation values with the passage of time),component Gx, and component Gy, horizontal movement detecting circuit 91and vertical movement detecting circuit 92 calculate horizontal movementamount Vx and vertical movement amount Vy of an image, respectively.Gradation disturbance prediction circuit 100 calculates equivalentgradation disturbance me according to gradient horizontal component Gx,gradient vertical component Gy, horizontal movement Vx, and verticalmovement Vy.

FIG. 17 shows the relation between movement vector V represented byimage movement components (Vx, Vy), and gradient component VG of vectorV Component VG is calculated by gradation disturbance prediction circuit100 of FIG. 16.

FIG. 18 explains an in-detail structure of gradation disturbanceprediction circuit 100. Arc-tangent functions converters 101, 102, andsubtracter 103 calculate angle θ, which is defined by movement vector Vand gradient direction G. The calculated value of angle θ undergoesconversion in cosign function converter 104. The result is furthermultiplied by the absolute value of the moving amount of image obtainedat absolute value calculating circuit 106. Movement component VGconverted into gradient of image is thus obtained. Having the structuresimilar to gradation disturbance evaluating circuit 10 of FIG. 1, table107 can predict the occurrence of the dynamic false contour.

With the structure above, the device can evaluate the movement of imageas an amount converted into the gradient of image and properly predictthe occurrence of the dynamic false contour. In this way, proper imagecorrection, and accordingly, an excellent image display can be realized.

Fourth Exemplary Embodiment

FIG. 19 is a block diagram of still another structure of the gradationaldisplay device of the present invention. In FIG. 19, like parts areidentified by the same reference marks as in FIG. 1. Output G fromgradient detecting circuit 3 and output B from time-varyinggradation-value detecting circuit 4 are fed into horizontal movementdetecting circuit 14, vertical movement detecting circuit 15, 45°-angledmovement detecting circuit 16, and 135°-angled movement detectingcircuit 17. Each output from circuits 14 and 15 is fed into movementamount calculating circuit 18. Receiving the result from circuit 18,final judge circuit 8 outputs final judge result k, which is fed intogradation correcting circuit 19.

Gradation correcting circuit 19 receives image signals from inputterminal 1. Circuit 19 is responsible for controlling gradationalcorrection for correcting the gradation values of incoming image anderror-variance. The methods of the gradational correction and theerror-variance are controlled by the outputs from horizontal movementdetecting circuit 14, vertical movement detecting circuit 15, 45°-angledmovement detecting circuit 16, and 135°-angled movement detectingcircuit 17, and final judge result k from final judge circuit 8. Thegradationally corrected image signals are then fed into sub-fieldgradation display device 13 for image display on the screen.

The magnitude of movement of an image, which is detected in the fourdirections, is used for control in gradation correcting circuit 19. Thecalculation of the magnitude itself of movement of an image can bederived from the two: the amounts of horizontal movement and verticalmovement. Receiving the two results, movement amount calculating circuit18 calculates the magnitude of movement. The magnitude is then sent tofinal judge circuit 8, where final judge result k corresponding to anecessary amount of gradational correction is determined.

Here will be given in-detail description of gradation correcting circuit19. Circuit 19 carries out gradational correction of incoming imagesaccording to a plurality of directions of movement amount of an image, aplurality of data on magnitude of the image, and final judge result kthat corresponds to a necessary amount of gradational correction.Gradation correcting circuit 19 employs the encoding method similar tothose shown in FIGS. 12 and 13.

FIG. 20 shows a typical structure of gradation correcting circuit 19.Circuit 19 contains, as shown in FIG. 20, adder 191, encoding circuit192, movement amount input terminal 193, output terminal 194, subtracter195, delay circuits 196 through 199, coefficient circuits 200 through203, and coefficient control circuit 204. The previously obtained dataon movement of image, i.e., the amounts of horizontal movement, verticalmovement, 45°-angled movement, and 135°-angled movement (i.e., a-d inFIG. 20) have been entered in coefficient control circuit 204. Receivinga, b, c, and d, coefficient circuits 200, 201, 202, and 203 calculatecoefficients EA, EB, EC, and ED, respectively. Each coefficient is usedfor signal calculation in delay circuits 196 through 199, and then fedinto adder 191. The process above forms an error-variance loop.

In the structure of FIG. 20, the signal fed into movement amount inputterminal 193 is responsible for the gradation control on the gradationvalues of incoming image signal. The encoding method shown in FIG. 13 iscarried out in encoding circuit 192 of gradation correcting circuit 19.

The incoming image signal is fed, with the number of the gradationlevels determined suitable for the movement amount of the image, to thedisplay device, whereby the dynamic false contour is effectivelysuppressed. At the same time, by virtue of the error-variance loop inthe structure, equivalent gradation values are maintained. The dynamicfalse contour can be suppressed by keeping the gradation levels to alimited number; an excessive limitation, however, can invite aninconveniency—the error-variance process increases noises, anddegradation in image quality may result.

FIG. 21 shows a typical coefficient distribution for error-variance.When pixel P undergoes gradational control, the difference between theinput signal and the display signal is distributed to the adjacent fourpixels: A, B, C, and D. Distribution coefficients EA, EB, EC, and EDtake, for example, the values shown in FIG. 22. A small movement of animage will not substantially cause the dynamic false contour—the devicedetermines the image as still image. In this case, distributioncoefficients EA, EB, EC, and ED take, as shown in FIG. 22, 7, 1, 5, and3, respectively. The values given to the coefficients should sum up to 1since the coefficients of error-variance are supposed to have adistributed portion of error; for purposes of inconvenience, the 16-foldvalue is employed in FIG. 22.

When an image shown on the screen moves in a specific direction, eachcoefficient of EA, EB, EC, and ED takes a different value according tothe moving direction shown in FIG. 22. The values in the table aredefined to each coefficient when the image noticeably moves in eachdirection; in the actual operation, the coefficients take values with acontinuous, or a step-by-step change according to the movement of theimage.

FIG. 23 illustrates the coefficient distribution, taking coefficient EAas an example. When the image is a still picture, coefficient EA takes7. When detecting the movement of the image, for example, in thehorizontal direction, the device gives 10 to coefficient EA according tothe movement. When the image moves in the vertical direction,coefficient EA is determined so as to gradually change from 7 to 0.Similarly, when the image moves in diagonal directions, coefficient EAgradually changes from 7 to 3.

FIG. 24 also illustrates the distribution, showing the relation betweenangle θ shown in FIG. 22 and movement of image. Supposing that an imagemoves in a direction having angle θ from the horizontal direction, FIG.24 shows the movement of image as a vector (where, the magnitude of themovement of the image is represented by m). FIG. 25 shows the transitionof coefficient EA, where values which coefficient EA can take in thetransition are interpolated from the values shown in FIG. 23. In thegraph, the horizontal direction on the screen is represented by θ=0. Thevertical axis of the graph represents values given to coefficient EA.Point P in FIG. 25 corresponds to point P in FIG. 24, and thecoefficient value is represented by EA.

The coefficient values of error-variance continuously vary, as describedabove, according to the moving amount, in direction and magnitude, ofimage with respect to the value defined in the still image. Therefore,the device can offer a smooth gradational correction according to themoving amount of image in direction and magnitude, thereby decreasingthe occurrence of the dynamic false contour and carrying out a propererror-variance.

As for other coefficients, for example, coefficient EB takes valuesshown in FIG. 26. Interpolating the values of FIG. 26, FIG. 27 shows thetransition of coefficient EB. FIGS. 28 and 29 show the values taken bycoefficients EC, ED, respectively. Each of coefficients EC and ED takesa transition (not shown) different from those of EA (FIG. 25) and EB(FIG. 27).

The present invention, as described so far, provides a gradation displaydevice employing the sub-fields capable of performing signal processingincluding the control of gradational correction and error-variance. Withthe device, excellent gradation display is obtained, with the occurrenceof the dynamic false contour decreased.

In the description above, considering an optical phenomenon of humaneyes, the coefficients of error-variance for the pixels located parallelto the moving direction of image are determined to have a relativelylarge value. Researchers say that when the viewer's eyes follow a movingobject on the screen, the amounts of emission by the pixels along themoving direction are perceived as a “visually amalgamated” amount on theretinas of the eyes. That is, it seems that the pixels aligned in thedirection parallel to the moving direction of image work equivalent toone pixel. Sharing a larger amount of error with the pixels in thedirection parallel to the movement reduces the amount of error-varianceassigned to the pixels insusceptible to the “visual amalgamation”, i.e.,the pixels aligned in a direction orthogonal to the movement of theimage. This can suppress an increase in noise in the error-variance.

Although the description of the embodiment introduces a linerinterpolation of coefficient values, it is not necessarily limitedthereto; a curvilinear interpolation using higher dimensional functions,or other continuous functions can be employed. Although the descriptiontakes an example of the gradation control in which the gradation valuesfalls into several steps; it is not limited to the number of thegradation steps. As a peculiar example, the gradational correction maynot control the number of the gradation values but the coefficients oferror-variance. The coefficients of error-variance described in theembodiment are not limited to those shown in the drawings; it will beunderstood that the same effect can be obtained by using othercoefficients, as long as the coefficients are determined inconsideration of the visually amalgamated effect in the moving directionof image.

As described above, the device of the present invention contains agradient detector for detecting a gradient of gradation values of pixelsin an image fed into the device; a time-varying gradation-value detectorfor detecting changes in gradation values of pixels with the passage oftime; an image detector for detecting the magnitude and direction ofmovement of an image to be entered according to the outputs from thegradient detector and the time-varying gradation-value detector; and asignal corrector for correcting signals of incoming image according tothe detected magnitude and direction of an image and a weight assignedto each sub-field so as to display proper image on the screen. Thedevice structured above detects the moving direction of image andlocates the area where the dynamic false contour is likely to occur.Therefore, the device can provide effective gradational correction,accordingly, excellent image display with proper gradationcharacteristics maintained, as well as effectively suppressing thedynamic false contour.

According to the present invention, the movement and gradient of theimage area being susceptible to the dynamic false contour can bedetected by a simple structure, whereby image display with high qualityis obtained, with the dynamic false contour properly suppressed. In thisway, the quality of image display of a gradational display deviceemploying the sub-fields can be improved.

INDUSTRIAL APPLICABILITY

According to the present invention, the movement and gradient of theimage area being susceptible to the dynamic false contour can bedetected by a simple structure, whereby image display with high qualityis obtained, with the dynamic false contour properly suppressed. In thisway, the quality of image display of a gradational display deviceemploying the sub-fields can be improved.

1. A gradation display device in which a TV field is divided into aplurality of sub-fields each of which has a predetermined weight ofluminance, the device comprising: a gradient detector for detecting agradient of gradation values of pixels in an incoming image; atime-varying gradation-value detector for detecting changes in thegradation values in the pixels with a passage of time; an image detectorfor detecting a magnitude and a direction of movement of the incomingimage according to outputs from the gradient detector and thetime-varying gradation-value detector; and a signal corrector forcorrecting signals of the incoming image according to the detectedmagnitude and direction of the image and the weight of luminanceassigned to each of the sub-fields so as to display proper image.
 2. Agradation display device in which a TV field is divided into a pluralityof sub-fields each of which has a predetermined weight of luminance, thedevice comprising: a smoothness detector for detecting smoothness ofgradation values of pixels in an incoming image; a gradient detector fordetecting a gradient of the gradation values of the pixels in theincoming image; a time-varying gradation-value detector for detectingchanges in the gradation values in the pixels with a passage of time; animage detector for detecting a magnitude and a direction of movement ofthe incoming image according to outputs from the gradient detector andthe time-varying gradation-value detector; and a signal corrector forcorrecting signals of the incoming image according to the detectedmagnitude and direction of the image and the weight of luminanceassigned to each of the sub-fields so as to display proper image.
 3. Thegradation display device of claim 1, wherein the device separatelydetects a horizontal component and a vertical component of a directionof movement of an incoming image, and converts gradient and movement ofthe image into a component in an direction of the gradient to provideproper signal correction.
 4. The gradation display device of claim 1,wherein the signal corrector not only controls correction of thegradation values of the incoming image but also controls error-variance.5. The gradation display device of claim 4, wherein the signal correctorcontrols the gradation values of the incoming image according to themagnitude of movement of the image and controls signal processing forthe error-variance according to a direction of the movement of theimage.
 6. The gradation display device of claim 2, wherein the deviceseparately detects a horizontal component and a vertical component of adirection of movement of an incoming image, and converts gradient andmovement of the image into a component in an direction of the gradientto provide proper signal correction.
 7. The gradation display device ofclaim 2, wherein the signal corrector not only controls correction ofthe gradation values of the incoming image but also controlserror-variance.